有关视频裁剪我在前面一篇博客中只是做了简单的说明，主要涉及到的知识准备在这篇博客中拿出来说一下。有兴趣的可以fork我的视频裁剪的项目源码，一起学习进步。

Github：https://github.com/iknow4/Android-Video-Trimmer

视频裁剪页如下图所示：

图上面的视频播放用是VideoView来实现的，比较简单。我主要说说视频帧的读取和视频时间的选取。

视频帧的获取可以通过Android提供的MediaMetadataRetriever类来实现。

MediaMetadataRetriever mediaMetadataRetriever = new MediaMetadataRetriever();

mediaMetadataRetriever.setDataSource(context, videoUri);

在MediaMetadataRetriever类中会调用到media_jni的这个库中的setDataSource()方法，最终使得我们可以获取视频的meta data，meta data是非常丰富的，足足有26种，足够满足我们的需求了。

/**

* The metadata key to retrieve the numeric string describing the

* order of the audio data source on its original recording.

*/

public static final int METADATA_KEY_CD_TRACK_NUMBER = 0;

/**

* The metadata key to retrieve the information about the album title

* of the data source.

*/

public static final int METADATA_KEY_ALBUM = 1;

/**

* The metadata key to retrieve the information about the artist of

* the data source.

*/

public static final int METADATA_KEY_ARTIST = 2;

/**

* The metadata key to retrieve the information about the author of

* the data source.

*/

public static final int METADATA_KEY_AUTHOR = 3;

/**

* The metadata key to retrieve the information about the composer of

* the data source.

*/

public static final int METADATA_KEY_COMPOSER = 4;

/**

* The metadata key to retrieve the date when the data source was created

* or modified.

*/

public static final int METADATA_KEY_DATE = 5;

/**

* The metadata key to retrieve the content type or genre of the data

* source.

*/

public static final int METADATA_KEY_GENRE = 6;

/**

* The metadata key to retrieve the data source title.

*/

public static final int METADATA_KEY_TITLE = 7;

/**

* The metadata key to retrieve the year when the data source was created

* or modified.

*/

public static final int METADATA_KEY_YEAR = 8;

/**

* The metadata key to retrieve the playback duration of the data source.

*/

public static final int METADATA_KEY_DURATION = 9;

/**

* The metadata key to retrieve the number of tracks, such as audio, video,

* text, in the data source, such as a mp4 or 3gpp file.

*/

public static final int METADATA_KEY_NUM_TRACKS = 10;

/**

* The metadata key to retrieve the information of the writer (such as

* lyricist) of the data source.

*/

public static final int METADATA_KEY_WRITER = 11;

/**

* The metadata key to retrieve the mime type of the data source. Some

* example mime types include: "video/mp4", "audio/mp4", "audio/amr-wb",

* etc.

*/

public static final int METADATA_KEY_MIMETYPE = 12;

/**

* The metadata key to retrieve the information about the performers or

* artist associated with the data source.

*/

public static final int METADATA_KEY_ALBUMARTIST = 13;

/**

* The metadata key to retrieve the numberic string that describes which

* part of a set the audio data source comes from.

*/

public static final int METADATA_KEY_DISC_NUMBER = 14;

/**

* The metadata key to retrieve the music album compilation status.

*/

public static final int METADATA_KEY_COMPILATION = 15;

/**

* If this key exists the media contains audio content.

*/

public static final int METADATA_KEY_HAS_AUDIO = 16;

/**

* If this key exists the media contains video content.

*/

public static final int METADATA_KEY_HAS_VIDEO = 17;

/**

* If the media contains video, this key retrieves its width.

*/

public static final int METADATA_KEY_VIDEO_WIDTH = 18;

/**

* If the media contains video, this key retrieves its height.

*/

public static final int METADATA_KEY_VIDEO_HEIGHT = 19;

/**

* This key retrieves the average bitrate (in bits/sec), if available.

*/

public static final int METADATA_KEY_BITRATE = 20;

/**

* This key retrieves the language code of text tracks, if available.

* If multiple text tracks present, the return value will look like:

* "eng:chi"

* @hide

*/

public static final int METADATA_KEY_TIMED_TEXT_LANGUAGES = 21;

/**

* If this key exists the media is drm-protected.

* @hide

*/

public static final int METADATA_KEY_IS_DRM = 22;

/**

* This key retrieves the location information, if available.

* The location should be specified according to ISO-6709 standard, under

* a mp4/3gp box "@xyz". Location with longitude of -90 degrees and latitude

* of 180 degrees will be retrieved as "-90.0000+180.0000", for instance.

*/

public static final int METADATA_KEY_LOCATION = 23;

/**

* This key retrieves the video rotation angle in degrees, if available.

* The video rotation angle may be 0, 90, 180, or 270 degrees.

*/

public static final int METADATA_KEY_VIDEO_ROTATION = 24;

/**

* This key retrieves the original capture framerate, if it's

* available. The capture framerate will be a floating point

* number.

*/

public static final int METADATA_KEY_CAPTURE_FRAMERATE = 25;

// Add more here...

了解这些meta 信息，就可以知道一个视频所能给我们提供的信息有哪些了。

在这里，我们是要获取视频的帧(Frame)，帧其实就是图片，一帧可能会有N多张图片组成。通过调用MediaMetadataRetriever类中的getFrameAtTime()获取视频的帧的Bitmap。

/**

* Call this method after setDataSource(). This method finds a

* representative frame close to the given time position by considering

* the given option if possible, and returns it as a bitmap. This is

* useful for generating a thumbnail for an input data source or just

* obtain and display a frame at the given time position.

*

* @param timeUs The time position where the frame will be retrieved.

* When retrieving the frame at the given time position, there is no

* guarantee that the data source has a frame located at the position.

* When this happens, a frame nearby will be returned. If timeUs is

* negative, time position and option will ignored, and any frame

* that the implementation considers as representative may be returned.

*

* @param option a hint on how the frame is found. Use

* {@link #OPTION_PREVIOUS_SYNC} if one wants to retrieve a sync frame

* that has a timestamp earlier than or the same as timeUs. Use

* {@link #OPTION_NEXT_SYNC} if one wants to retrieve a sync frame

* that has a timestamp later than or the same as timeUs. Use

* {@link #OPTION_CLOSEST_SYNC} if one wants to retrieve a sync frame

* that has a timestamp closest to or the same as timeUs. Use

* {@link #OPTION_CLOSEST} if one wants to retrieve a frame that may

* or may not be a sync frame but is closest to or the same as timeUs.

* {@link #OPTION_CLOSEST} often has larger performance overhead compared

* to the other options if there is no sync frame located at timeUs.

*

* @return A Bitmap containing a representative video frame, which

* can be null, if such a frame cannot be retrieved.

*/

public Bitmap getFrameAtTime(long timeUs, int option) {

if (option < OPTION_PREVIOUS_SYNC ||

option > OPTION_CLOSEST) {

throw new IllegalArgumentException("Unsupported option: " + option);

}

return _getFrameAtTime(timeUs, option);

}

这里有两个参数：

timeUs 表示每一帧的开始时间位置，给定的时间的帧不一定存在，但是系统会将临近的一帧返回给我们。

option 参数表示获取帧的方式

以下是我调用的代码块，代码块是放在单独的线程中执行的。

try {

MediaMetadataRetriever mediaMetadataRetriever = new MediaMetadataRetriever();

mediaMetadataRetriever.setDataSource(context, videoUri);

// Retrieve media data use microsecond

long videoLengthInMs = Long.parseLong(mediaMetadataRetriever.extractMetadata(MediaMetadataRetriever.METADATA_KEY_DURATION)) * 1000;

long numThumbs = videoLengthInMs < one_frame_time ? 1 : (videoLengthInMs / one_frame_time);

final long interval = videoLengthInMs / numThumbs;

//每次截取到3帧之后上报

for (long i = 0; i < numThumbs; ++i) {

Bitmap bitmap = mediaMetadataRetriever.getFrameAtTime(i * interval, MediaMetadataRetriever.OPTION_CLOSEST_SYNC);

try {

bitmap = Bitmap.createScaledBitmap(bitmap, thumb_Width, thumb_Height, false);

} catch (Exception e) {

e.printStackTrace();

}

thumbnailList.add(bitmap);

if (thumbnailList.size() == 3) {

callback.onSingleCallback((ArrayList) thumbnailList.clone(), (int) interval);

thumbnailList.clear();

}

}

if (thumbnailList.size() > 0) {

callback.onSingleCallback((ArrayList) thumbnailList.clone(), (int) interval);

thumbnailList.clear();

}

mediaMetadataRetriever.release();

} catch (final Throwable e) {

Thread.getDefaultUncaughtExceptionHandler().uncaughtException(Thread.currentThread(), e);

}

子线程会在每截取到三帧之后将数据上报给主线程，主线程拿到数据之后会进行ui的更新操作，将bitmap显示出来。

当视频源时间比较长的时候，我们会截取到很多帧图片，所以需要将帧图片可以左右滑动。项目中用到的控件是：VideoThumbHorizontalListView (可水平滚动的ListView)

videoThumbListView.setOnScrollStateChangedListener(new VideoThumbHorizontalListView.OnScrollStateChangedListener({

@Override

public void onScrollStateChanged(ScrollState scrollState, int scrollDirection) {

if (videoThumbListView.getCurrentX() == 0)

return;

switch (scrollState) {

case SCROLL_STATE_FLING:

case SCROLL_STATE_IDLE:

case SCROLL_STATE_TOUCH_SCROLL:

if (scrolledOffset < 0) {

mScrolledOffset = mScrolledOffset - Math.abs(scrolledOffset);

if (mScrolledOffset <= 0)

mScrolledOffset = 0;

} else {

if(PixToTime(mScrolledOffset + SCREEN_WIDTH) <= mDuration)//根据时间来判断还是否可以向左滚动

mScrolledOffset = mScrolledOffset + scrolledOffset;

}

onVideoReset();

onSeekThumbs(0, mScrolledOffset + leftThumbValue);

onSeekThumbs(1, mScrolledOffset + rightThumbValue);

mRangeSeekBarView.invalidate();

break;

}

} );

我会去监听水平ListView的左右滚动事件，以获取它的滚动距离，从而算出并显示当前截取到的视频时间的长度。这里我们需要将视频总长度与屏幕的宽度做一个计算，形成一个映射关系，这是一个比较麻烦的事情，处理不好，会导致在ListView滚动过程中时间显示不准确的问题，哈哈，我的项目就存在着这样的bug。

这个项目只是一个粗糙的项目，页面上会存在一些小问题，不过可以拿来简单的学习一下。

欢迎有兴趣的同学star和fork。

Github：https://github.com/iknow4/Android-Video-Trimmer

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